It's the music all the way as far as I'm concerned, but I'll add that to be a successful tribute, I think you need to put a bit of effort in regarding the look as well.
There are tributes that overload on the image to mask their deficiencies in their playing capabilities. I won't name names, but for some reason most audiences seem oblivious.
It's a constant frustration of mine as I'm in three different tributes myself.
To answer the original question, it's the bands that nail both that really fly